I don't think either would make a good film. Bioshock would have a better chance, because there's atmosphere, a very unique mythology and a strong visual style. But the core philosophy of the game and story, revolve around the very nature of playing a game.

Half Life doesn't have any of those and its strengths in the story department wouldn't convert well to film at all. For a start you'd have to get rid of the protagonist (surely there'd be outcry if someone made Freeman visible and have a personality? That's the opposite of the point of Freeman), and then you've got quite a tight arc of the story where Freeman is the hero, so how would they even work out? The best thing they could do is a MGS2 style story, where the protagonist is a little guy with a little storyline taking place in the setting of a big story and Freeman is constantly mentioned and shadowed but never appears, in an almost mythological way.

That could work and would be pretty interesting actually, but it would be hard to convince anyone who hasn't played the game that it isn't the most generic of generic alien plots/setting with the typical black suited police thugs and puppet government to boot. Aliens invade! Yay unrealistically powered and lovable hero! Zombies! Go pleasant female companion! You've got to play the game to understand there's more to it than.

Also if I were to try and make a Half Life film, I'd be too tempted to spend most of my time making seesaw puzzle jokes

I myself am quite pleased to see Metroid Prime's story recognized...as you might be able to gather from my name. Much like Half-Life 2, actually, its story is fairly unremarkable, but it's presented extremely well. Metroid Prime makes you a detective. Every piece of backstory and history you locate is an accomplishment, another tool to help you crack the mysteries of Tallon IV. By not telling you the story outright, the game makes that story elusive and thus compelling.

Or...something like that. I dunno, it's 2:15 in the morning, I have no idea what I'm typing.